MBMixer
Hardware description:
This project is based on MidiBox hardware, and the MIOS
operating system. Please see http://www.ucapps.de for more
information on these.

This project may include or leave out a lot of options to
suit the individual using it. I will describe the hardware
used in the original reference design, but I'll try to show
which parts are optional as I go.

Required parts:
MIDIBox Core module.
Up to 16 X PGA2311 or 8 X PGA4311 chips.
Bi-Polar power supply providing 2 +5Volt regulators and 1 -5Volt regulator.
Summing Amps to mix all the outputs. I'm using a NE5532 based circuit.

Optional parts:
1 encoder and DINX1 module, 
OR
as many encoders as you'd like, with DINX4 module for every 16 encoders.
LCD display. Reference display is 2 lines X 20 characters.
preferred display will have vertical bars at ASCII positions 16 to 24(d)
to support the stereo bargraph display. I'm using a Noritake VFD display
with the character set CG57103. If your display does not support this,
you'll still have the dB display available.
Input buffers may be included, especially if you're not sure of the impedance
of the input signal. I'm using a NE5532 dual Op-Amp for each input pair. Other
reference designs are provided.

Module Schematics:
To allow for the most options, I'll provide the circuits in modules, so that
each user can choose only the parts they need for their own design.
Circuits for the Core Module and the LCD display can be found at ucapps.de,
listed above.

Power Supply:
This project requires a "Bi-Polar Power Supply". Simply put, this means it
has both positive and negative voltages. We will create two separate +5 volt
supplies for the PGA chips. One is for the Audio section, and a separate 
regulator will feed the digital section. This is to keep the digital switching 
noise out of the audio circuits. I also feed unregulated DC to the MIDIBox,
it will use its own regulator for power.

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The power supply circuit starts with a fuse and power switch. After these is a
16VAC center tapped transformer. We ground the center tap, and feed the two power
windings to a full-wave bridge rectifier (4 diodes). The rectifier outputs are
then filtered with a couple big capacitors to reduce the ripple. I used 4700 uf
at 35VDC, but anything big should work, as long as the voltage rating is sufficient.

Now we have positive and negative power, unregulated. Mine is about +9.5 and -10.0
volts. I take the +9 and Ground from here over to the MIDIBox, and tie them
directly to Capacitor C5, (after the bridge). Watch the polarity!
I can't connect before the bridge or we wouldn't all have the same ground.
Back on the power supply, I connect my V+ to two 7805 regulators, and I connect
V- to a 7905 negative regulator. Be careful, the pinouts are different for positive
and negative regulators. Check the regulator outputs for +5 or -5, and wire those
to a connector for other boards to plug into.
Double check for two clean +5 and a -5. The power supply is now complete.

PGA3211/4311 chips:
Each PGA2311 chip can control two audio channels (one pair of stereo).
Each PGA4311 chip can control four audio channels (two stereo pairs).
The PGA2311 chips are available in DIP or SMT. DIP is MUCH easier to build by hand.
The 4311 chips are SMT only. I suggest either using a pre-made circuit board, or
using the SchmartBoard EZ prototyping PCBs. The correct model for the PGA4311 chip
is 202-0004-01. I was impressed with how well these boards work. These are a great
way to begin soldering surface mount parts.
The datasheet for the PGA chips shows the connection diagram. In my board these 
connect to J10.

The PGA Chip Select signals (CS) come from "J10-SO", Pic PORTC,5

The PGA Serial Clock signals (SCLK) come from "J10-SC", Pic PORTD,3

The first PGA Serial Data In signal (SDI) comes from "J10-RC", Pic PORTC,4
The SDO of the first PGA goes to the SDI of the next chip...

These pin assignments can be changed with #define statements in pga.inc.

Input Buffers:
The datasheet shows the inputs connecting directly to the PGA chips. This may be
OK if you know the impedance of the incoming signals. If you're not sure what will
be plugged in, you may want to buffer the incoming signals. I'm using a simple
non-inverting buffer made with NE5532 op amps. Each input is connected through
a 10uf capacitor, then a 47K resistor to ground. The signal is connected to the
+ input of the op-amp, and the output is tied back to the - input. The op-amps
are powered with the same + and - 5 volt power as the analog side of the PGA chips.
Even if you do not buffer the inputs, you  should consider keeping the 10uf cap and
a resistor to ground, to remove any DC bias from your audio signal.

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Input stages for balanced, unbalanced, high or Low level, Phono(RIAA), and phantom
power support may be found at http://users.otenet.gr/~athsam/Audio_mixer_6_ch_ENG.htm
While those examples use higher voltages, there is no advantage to using more than
the +-5 volts available, since the PGA chips cannot exceed that voltage anyway.

The PGA outputs may be summed (mixed) together with another NE5532. Simply connect
each output with 10K to ground and 10K to the negative input of a 5532 amp. Connect
a 10K from the output of the amp to the same mixing point. Connct the + input to 
ground. The output of the 5532 should now be all the input signals combined (inverted).

As you connect the PGA chips, the first chip connected will be channel 1, and channel
numbers will go up from there.

Encoder(s):
The reference design uses a single pushbutton encoder. A Din X1 is enough to connect 
this. Bigger/More DIN boards will be necessary for more encoders. These DIN modules 
will connect to J9 of the core module. You'll need to configure MIOS to read the 
encoders. This is done in the MIOS_ENC_PIN_TABLE part of the mios_tables.inc file.

If you have more than one encoder, some software changes to support more encoders 
may be needed.


Pictures:

This is the Summing Amp, A couple pairs of input buffers, and a PGA4311 on a Schmartboard.


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The Back Panel. Probably the prettiest part of my little mixer

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The dB Display , This was written by Pilo.

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